Field of the Invention
This invention relates to a drive mechanism/transmission mechanism for a bicycle.
Background
Bicycles commonly use roller chain transmissions, with multiple chain wheels and sprockets together with derailleur shifting mechanisms, to connect the pedals to a wheel. Roller chains wear and stretch in use and the sprockets, with which they engage, wear until they break, slip or otherwise fail, requiring expensive replacement. To minimize wear the mechanism needs regular cleaning and lubrication. This is especially the case for bicycles used off-road where contamination by abrasive matter is unavoidable. Roller chain mechanisms also suffer from a number of other problems, for example, the chain can contaminate and snag the rider's clothing, the chain can become dislodged from the sprockets, shifting cannot readily take place when stationary and the shifting mechanism itself is prone to contamination and vulnerable to damage. Also the externally lubricated chain of a folding bicycle is likely to contaminate the user's clothing when the folded bicycle is carried.
The large pitch of roller chains, and the resulting small number of teeth on each sprocket, results in inconsistent ratio changes. This gives an inconsistent feel to shifting and requires a greater number of sprockets to be used to cover a given range of ratios while maintaining close ratio shifts.
The crank shaft carrying the pedals of a roller chain transmission bicycle must be mounted on the frame of the bicycle in front of the rear wheel so as not to interfere with the wheel's rotation. This, combined with the circular motion of the pedals, results in a pedaling action where the downward portion of motion of the pedal is short in comparison with the full motion, and occurs some distance in front of the rider's center of mass. Roller chain transmission bicycles with a short wheel base, as used in racing, suffer from toe overlap, where the rider's feet can interfere with the front wheel when steering.
Bicycles often incorporate suspension systems to improve riding on rough terrain. For a roller chain bicycle, the pivot point for the rear suspension on the frame must be substantially at the center of the crank shaft, to minimize variations in the center distance between the chain wheel on the crank shaft, and the sprockets on the rear wheel, as the suspension operates. The need to co-locate the crank shaft and suspension pivot mounting points, both of which require high strength and stiffness, can result in additional complexity, cost and weight.
Early bicycles, generally predating roller chain bicycles, used a variety of treadle mechanisms. One example used a pair of levers mounted on cranks connected to the steering, front, wheel with a link to the forks at their forward ends and a pedal at their rear ends. This mechanism does not have the same disadvantages as a roller chain transmission, but has a number of other disadvantages. This bicycle had no gearing, with the driven wheel rotating at the same frequency as the pedals thus limiting speed for a given wheel size. This bicycle did not have a free wheel mechanism so the rider was unable to rest his legs when moving, and it was not possible to set the position of a pedal to assist starting to ride. As the pedal and drive mechanism was mounted to the front wheel, forces from the rider's legs would interfere with the steering of the bicycle.
It is an object of the present invention to address the above problems.